Transglucosidases (EC.2.4.1.24, 1,4-alpha-glucan 6-alpha-glucosyltransferase) are D-glucosyltransferase enzymes that catalyze both hydrolytic and transfer reactions on incubation with alpha-D-gluco-oligosaccharides (1951, Pazur and French, J. Amer. Chem. Soc. 73:3536). Maltose is the most preferred substrate for transglucosylation reactions with this enzyme. Transfer occurs most frequently to HO-6, producing isomaltose from D-glucose, or panose (6-O-alpha-glucosyl maltose) from maltose. Transglucosidase can also transfer a glucosyl residue to the HO-2 or HO-3 of another D-glucosyl unit to form Kojibiose or Nigerose. This enzyme can further transfer a D-glucosyl unit back to HO-4 to reform maltose.
As a result of transglucosylation reactions with transglucosidase, malto-oligosaccharide residues are converted to isomalto-oligosaccharides (IMO) containing a higher proportion of glucosyl residues linked by alpha-D-1,6 glycosidic linkages from the non-reducing end. IMO sugars are used in many food and beverage formulations in Asia. Brier et al. (U.S. Patent Appl. Publ. No. 2003/0167929) disclosed using transglucosidase to produce IMO from barley wort.
Poulose et al. (U.S. Patent Appl. Publ. No. 2008/0229514) disclosed using transglucosidase to degrade polysaccharides such as xanthan and guar gums. Xanthan gum comprises a cellulosic backbone in which alternate glucoses are 1,3-linked to branches containing mannose and glucuronic acid. The backbone of guar gum comprises beta-1,4-linked mannose residues to which galactose residues are alpha-1,6-linked at every other mannose.
Lantero et al. (U.S. Pat. No. 5,770,437) disclosed using a transglucosidase to degrade sucrose, melezitose and trehalulose. These sugars comprise glucose linked to fructose via 1,2- (sucrose), 1,3- (melezitose), or 1,1-(trehalulose) linkages.
Although various hydrolytic activities of transglucosidase have been disclosed, this type of enzyme is generally considered to be an alpha-glucosidase, given its ability to hydrolyze alpha-linkages between two glucosyl residues. For example, transglucosidase is associated with having maltase activity (hydrolysis of the alpha-1,4 glycosidic link between the two glucosyl residues of maltose), which is a type of alpha-glucosidase activity.
Notwithstanding the foregoing disclosures, surprisingly, it has now been found that alpha-glucosidases such as transglucosidase (EC 2.4.1.24) can hydrolyze alpha-1,3 and alpha-1,6 glycosidic linkages of glucosyl-glucose. Alpha-glucosidase enzymes are disclosed herein as being useful for degrading disaccharides and oligosaccharides containing glucosyl-alpha-1,3-glucose and glucosyl-alpha-1,6-glucose.